Dynamic Rupture Processes on a Dipping Fault, and Estimates of Stress Drop and Strength Excess from the Results of Waveform Inversion

SUMMARY The dynamic rupture processes on a dipping fault in a horizontally layered medium are investigated on 3-D, spontaneous shear-crack models. The wave equations for the 3-D space are solved numerically by a finite difference method under appropriate boundary conditions. The displacement components right above and right below the dipping fault plane have been obtained by solving the boundary conditions across the fault. It was found that the final fault slip on the hanging wall side is appreciably larger than that on the foot wall side, and that their amount is quite sensitive to the fault depth and the elastic heterogeneities of the medium, particularly of the existence of low-velocity surface layers. On the basis of the above model, the dynamic rupture process of the 1961 Kita-Mino earthquake in central Japan has been investigated with constraints of the fault parameters derived from the results of waveform inversion analysis. For this purpose, a locking fracture criterion was introduced for rupture propagation, from which a lower bound of the maximum shear stress before the rupture and hence of the strength excess has been estimated. The dynamic stress drop has also been evaluated by repeated iterations to minimize the difference between the dynamic and kinematic fault slips. The results revealed quite heterogeneous distribution of strength excess, indicating large values near the SW section of the fault at shallow depths and in the central bottom section, in contrast to small values at mid-depths in the NE section around the rupture nucleus zone. The distribution of stress drop was also found to be heterogeneous, with the existence of high-stress drop zones in the NE, shallow and bottom sections and a low-stress drop zone at most of mid-depths except in the SW section. These results suggest that the dynamic rupture of this earthquake initiates at a small nucleus zone with low-strength excess and small-stress drop, then broke the shallower and deeper fault sections with moderate-strength excess and high-stress drop, and finally ruptured barrier zones with high-strength excess. These zones may be interpreted as weak and strong asperities and barriers, respectively.